The present invention comprises a method for sizing paper and similar products and further describes new classes of chemical materials suitable for sizing these products. The method and the new sizing compositions are particularly well suited for sizing in the pH range between about 6.5 and 10.5.
Sizes are used in virtually all finished paper products their primary purpose is to reduce the rate at which the paper sorbs moisture. Sizings fall into two categories: internal fiber treatments and surface treatment of papers. Internal sizes are added to the papermaking stock at some point before the fibers are formed into a web on the paper machine. In surface treatment, sizes are applied as a coating at some point in the dryer section of the paper machine. Sizes for surface treatment are composed of a great variety of natural materials, synthetic resins and mixtures of the two types of materials. They are important to impart liquid holdout in such applications as milk cartons or meat wrappings where the paper is in actual contact with liquids and must prevent their transmission for extended periods of time. They are also used on coated papers to give ink holdout and prevent blurring of printed material. Most papers which are surface treated are also internally sized. This enhances holdout and increases the resistance of the web to the penetration of water while the surface size is applied at the size press in order to minimize sheet breaks.
For several hundred years ink holdout was the only consideration which necessitated sizing papers. Animal derived gelatins and perhaps a few naturally derived gum materials served very successfully when most papers were hand made. With the advent of paper machines which increased the production rate by orders of magnitude, other materials had to be found. Rosin emulsion and soaps, precipitated by alum, came into use about 1830 and have been so successful that they are still the predominantly used internal sizing material. Despite a spectrum of other sizing compositions found in the patent literature, only two besides those based on rosins have achieved even limited commercial success. These are alkyl substituted ketene dimers and alkenyl substituted succinic anhydride. These latter materials are much more expensive than rosin but may successfully be used in the pH range between 6 and 8. Because of its chemical nature, rosin is not generally considered to be an effective sizing material above about pH 6.5 and the great bulk of it is used in the pH range of 4.5 to 5.5.
There are both advantages and disadvantages to forming papers under acidic conditions. One advantage is slightly faster drainage on the wire section of the paper machine. The disadvantages are numerous. Equipment corrosion is one. The relatively short life of paper products made with acidic sizing is another. Deterioration of the paper made under acidic conditions is causing millions of books and other documents to slowly turn to dust before the eyes of their owners. Libraries are spending extensive sums to neutralize the acidity in their books. But due to the expanse and time involved, only the most valuable volumes made from acidic paper can be treated.
In addition to the problems caused by corrosion and paper deterioration, there are other reasons why sheeting at neutral to slightly alkaline conditions is desirable. The papermaker has more choice to choose component materials and may use fillers such as calcium carbonate which are not compatible with acidic systems. However, the expense and difficulty of use of ketene dimer and succinic anhydride based products has greatly limited their use and the time is ripe for new products or techniques.
Conventional sizing theory has been predominantly developed around rosin. Traditional belief holds that sizing occurs when an ambipathic material such as rosin is distributed as uniformly as possible on the fibers with a material such as alum. It is implicit that the sizing material must be oriented with the hydrophobic "tail" outward. It has been assumed that the alum in the system forms a bridge between anionic carboxyl groups of the rosin and the anionic cellulose fibers. When heated in the dryer section of the paper machine, the rosin, under the influence of heat, is presumed to "flow" over the fibers to ensure thorough and uniform distribution. During the application of heat it is assumed that an aluminum ester of rosin is formed that serves to orient the rosin on the fiber with the hydrophobic tail outward.
It may be noted at this point that rosin size is normally available in one of two forms: as an emulsion where the rosin is in the form of rosin acid (acid size) or in solution as the sodium soap (neutral size). Normally both of these forms of size are chemically modified. Typically they will be reacted with formaldehyde to decrease the tendency of the rosin to crystallize and often they are reacted with either fumaric acid or maleic anhydride to improve their efficiency. In neutral sizes from 80-100% of the rosin is in saponified form. Only about 5% of the rosin is saponified in acid sizes. This small amount is sufficient to enable formation of a self-stabilized emulsion without significant need for other emulsifying agents. When neutral rosin soap is used as the sizing agent, it must be well distributed through the system before alum is added. With sizes of this type, it is believed that sizing predominantly occurs at the wet end of the machine with little further development of sizing during drying. In the case of rosin acid sizes, uniform wet end distribution is essential, but sizing continues to develop as the paper passes through the dryer section. Marton and Marton, Proceedings, Tappi Papermakers Conference, pp. 15-24 (1982) present a useful discussion of the difference between rosin acid and rosin soap sizes and the presumed mechanism by which they contribute to sizing.
The exact physical chemical mechanism by which sizing molecules bond to cellulosic fibers has been the subject of much debate and there is not a general consensus among experts. The mechanism explained earlier for rosin soap sizes has been fairly generally accepted. Controversy still exists as to the mechanism by which rosin acids, alkyl ketene dimers and alkenylsuccinic anhydride impart sizing. The manufacturers of these sizes generally support the idea of chemical reaction with the cellulose to form covalent bonds. Others insist that this is unlikely and that other attachment mechanisms prevail. An unpublished M.A. Thesis by Lund (University of Washington, Seattle, 1983) gives convincing evidence that alkyl ketene dimer sizes do not react with the fiber to form covalent bonds. These latter two materials differ in one important respect. Alkenylsuccinic anhydride materials develop sizing at the wet end of the machine and little change occurs through the dryer section. Contrary to this, alkyl ketene dimers continue to develop sizing as the paper is heated in the dryer section.
The question of what actually occurs during sizing appears to be even more complex as the process is carried out in near neutral to slightly alkaline environments in the range between pH 6 and 8. Here it is common to add small amounts of polycationic materials to the system. Cationic starch is one such material and polycationic materials normally used as wet strength resins are another. Among the latter group are polyalkylene polyamine epichlorohydrin compositions and cationic urea-formaldehyde condensation products. These are normally added with the size or subsequent to the addition of the sizing material although practice in this regard is not uniform. It is known that these materials help in some way, but the most generally held belief is that they serve to break the sizing emulsion and contribute a slightly cationic charge to the sizing particles so that they will be attracted to and attach to the anionic cellulose fiber surfaces. Various writers refer to these cationic materials, as well as to alum, as fortifying agents, retention agents, or fixatives.
The patent literature on sizing agents is extensive. Despite the many new compositions that have been described, only the three discussed earlier have achieved significant commercial importance. However, the patent literature is informative as to various materials which have been proposed as sizing and the manner in which these materials are presumed to operate. As one example, British Pat. No. 1,255,829 describes a rosin acid size having 80-98% free acid. This material is used with up to 0.5% alum based on fiber with up to 0.25%, based on fiber, of a water soluble cationic polyamide having a molecular weight in excess of 1,000. This system is said to be usable in the pH range of 6-7.5, unusually high for a rosin based size. The reason the system works at this high pH is that the size is added and precipitated before it can be fully neutralized. The additives can be introduced in any order, either with the size, before the size or after the size. Preferred practice would be to add the cationic material after the size and alum have been introduced into the system.
A technical leaflet by the firm BASF, Charlotte, N.C. describes fixing agent FP as a cationic synthetic resin particularly suitable for fixing rosin size in neutral media. This material, believed to be a polyethyleneimine-type, is added to the paper stock prior to the addition of size and alum.
Kulick, U.S. Pat. No. 3,526,524 describes a rosin paste size which includes a water soluble cationic polyalkylenepolyamine. The cationic material is stated to increase the effectiveness of the rosin.
In Canadian Pat. No. 1,008,609, Strazdins teaches that a polyvinylcycloamidine can be used to increase the effectiveness of a conventional rosin-based size. The polymeric material, alum, and size can be added to a pulp slurry in any order or premixed except that the alum and size must be added separately to avoid premature precipitation.
Harding et al, U.S. Pat. No. 4,505,775, teach a process for preparation of cellulosic fibers made cationic by reaction with a modified epichlorohydrin-dimethylamine reaction product. These fibers are said to have less tendency to repel anionic additives and to make possible the use of sizes such as alkyl ketene dimers under less acidic conditions.
Kowatani et al, U.S. Pat. No. 4,576,680, describe new sizes based on alkenylsuccinic anhydride. These inventors note that polyamide polyamine resins are useful as sizing "fixing agents." The inventors are somewhat unclear as to how these materials are used, but they are apparently added to the pulp slurry prior to the addition of the sizing material.
Japanese Kokai 53[1978]-45403 teaches sizing agents based on hydrocarbon O-(substituted carbonyl) oxime derivatives. Polyethyleneimine can be used both as an emulsifier and may be added to the pulp slurry subsequent to the addition of the sizing. These sizes presumably work by an acyl transfer mechanism to attach a long chain hydrophobic portion of the molecule to the cellulose hydroxyl groups.
Beck, U.S. Pat. No. 3,900,335, discloses N,N'-alkyl substituted aspartimide sizing compositions. Common wet end additives such as polyacrylamides and alum may be used with the sizing material. No order of addition of these adjuncts is specified.
In U.S. Pat. No. 3,993,640, Pickard et al describe cyclic N-substituted imide sizing agents in which the N-substituted group is preferably an electron withdrawing group; e.g., a long chain acyl group. The composition presumably imparts sizing by an acyl transfer mechanism to attach the long chain acyl group to the cellulose. Aqueous emulsions of the sizing agents preferably contain a retention aid such as a polyacrylamide. A goal of the invention is to provide a size which is chemically tied to the cellulose by covalent bonding.
In U.S. Pat. No. 3,726,822, von Bonin et al. disclose anionic paper sizes based on succinimide-type compounds mixed with polymeric latices. A retention aid such as a condensate of dicyandiamide and formaledhyde is added to the pulp slurry prior to adding the size. This material may also be applied as a surface size to an alum treated paper.
Bowman and Cuculo, Textile Res. J., 45:766-722 (1975), in discussing the reaction of phthalamic acid with cellulose, note that the phthalimide is formed in a competing reaction which lowers the efficiency of the esterification. These compounds are analogs of succinic anhydride sizes. The above observation would speak against the usefulness of cyclic imide compounds as sizing agents.
The documents cited above are intended to be exemplary rather that fully inclusive of the literature on various sizing materials. The sizing materials described in them are primarily types designed for use in the neutral range and with which a polycationic materials is employed or recommended as a sizing adjunct. It is worthy of note that there is no unanimity of opinion as to when or where the adjacent should be added. Different inventors make different recommendations even though the sizing materials may be chemically similar.
What has been lacking in the field of paper sizing is a full and clear understanding of the mechanism by which sizing compounds are transferred to and retained by the fiber and the mechanism by which sizing further develops in the dryer section of the paper machine. The present inventors now believe they have discovered and understand these mechanisms. This knowledge has, in turn, led to a definition of chemical criteria for sizing compositions, especially those to be used in the near neutral and alkaline ranges. Of equal importance, the knowledge of how sizing is transferred and developed has led to new process criteria which are generally applicable to a wide spectrum of heretofore unsuspected and undescribed paper sizing compositions.